1. Field of the Invention
The present invention relates to a binarization circuit, and, more particularly, to a binarization circuit suitable for use in an information reproduction system which can optically read two-dimensional code patterns from information storage media such as a paper sheet on which so-called multimedia information including, e.g., audio information such as speech and music, image information obtained from a camera, a video device, and the like, and digital code data obtained from a personal computer, a wordprocessor, and the like are recorded as optically readable two-dimensional code patterns, thereby reproducing the original multimedia information.
2. Description of the Related Art
Conventionally, various media such as a magnetic tape and an optical disk have been known as media for recording speech and music. However, even if each medium is duplicated in large numbers, its unit price is high to a certain degree, and a large space is required for storing them. When a person needs to give a speech-recorded medium to another at a remote location, this person cannot avoid a cumbersome, time-consuming procedure to accomplish it regardless of whether the medium is mailed or handed. The same is true for so-called multimedia information including, e.g., image information obtained from a camera, a video device, and the like, and digital code data obtained from information processors such as a personal computer and a wordprocessor, other than audio information.
EP 0,670,555 A1 corresponding to U.S. patent application Ser. No. 08/407,018 discloses, as a solution to the above shortcoming, a system which can transmit multimedia information including at least one of audio information, image information, and digital code data via a facsimile, and records image information that can be duplicated in large numbers at a low cost, i.e., a plurality of dots as coded information arranged in a two-dimensional code pattern, on information storage media such as a sheet of paper, and a system for reproducing the recorded information.
In the reproduction system disclosed in the European patent publication, a binarization processing section adaptively performs binarization on an image signal, acquired by sensing the mentioned two-dimensional code pattern, while the threshold value of the image signal is determined by a threshold value determination circuit. As such a binarization processing section, a binarization circuit disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 59-61383, is known. This binarization circuit acquires the maximum value and minimum value of the previous frame of digital data converted by an A/D converter, computes a threshold value from those two values, and performs binarization using this threshold value as the threshold value for the current frame.
According to the reproduction system disclosed in the aforementioned European patent publication, the surface of a sheet as a recording medium is located close to an illuminator and an image pickup section, the image signal acquired from the image pickup section is greatly affected by the regular reflection and includes noise. Further, the image pickup section may have defective pixels, which would appear as noise. Such noise disables the binarization processing section disclosed in the aforementioned Japanese patent publication to accurately acquire the maximum value and minimum value, with the result that binarization will not be executed with the proper threshold value.